Electrochemical autothermal reformer

ABSTRACT

An electrochemical autothermal reformer (EATR) provides hydrogen. The EATR includes an autothermal reformer region, a reformer anode supply region, and a composite membrane layer separating the reformer anode from the autothermal reformer region. The composite membrane layer includes a mechanically stable porous ceramic support member with a thin gas permeable ceramic substrate layer overlaying the support member. Overlaying the substrate layer is a first thin metallic catalyst layer which promotes the dissociation of H2 to 2H++2e-. Overlaying the first catalyst layer is a metallic oxide layer capable of conducting 2H++2e- at elevated temperatures. Overlaying the metallic oxide layer is a second thin metallic catalyst layer which promotes the recombination of 2H++2e- to H2.

BACKGROUND OF THE INVENTION

This invention relates to a reformer that processes a hydrocarbon fuelto separate and provide hydrogen from the hydrocarbon fuel, and inparticular, to a composite membrane for an electrochemical autothermalreformer (EATR).

A "reformer" is a known device for converting hydrocarbon fuels tohydrogen, in which a hydrocarbon fuel is mixed with air and with orwithout steam to convert the mixture to hydrogen, carbon monoxide,carbon dioxide, water, and impurities. An autothermal reformer usesfuel, air, and steam. Since most known reformers are adversely sensitiveto the presence of impurities, impurities such as sulphur are generallyremoved from the fuel before entering the reformer. An electrochemicalautothermal reformer combines the principles of electrochemical hydrogenseparation with those of an autothermal reformer. The purpose of theelectrochemical autothermal reformer is to effect the removal ofhydrogen produced from the reaction zone of the reformer so as to drivethe reforming reaction to completion by separating or selectivelyextracting the hydrogen component from the rest of the product mixture.

For example, the principal reactions in a natural gas or "methane"reformer such as an autothermal reformer are:

    ______________________________________                                        CH.sub.4 + H.sub.2 O → CO + 3H.sub.2                                                          Reforming                                              CO + H.sub.2 O → H.sub.2 + CO.sub.2                                                           Shift                                                  CH.sub.4 + 2O.sub.2 → CO.sub.2 + 2H.sub.2 O                                                   Combustion                                             ______________________________________                                    

In an autothermal reformer, the exothermic combustion reaction is usedto drive the endothermic reforming reaction. The shift reaction ismildly exothermic. If hydrogen is abstracted or removed from thereaction zone of the autothermal reformer, then by LeChatelier'sprinciple, the reforming and shift reactions are driven to completion.Accordingly, the fuel processing in an EATR is greatly simplified sinceshift converters and selective oxidizers are not required downstreamfrom the fuel processor.

Marianowski et al. (U.S. Pat. No. 4,810,485) teaches a hydrogen formingprocess and apparatus wherein one side of a hydrogen ion porous andmolecular gas non-porous metallic foil is contacted with mixed gasescomprising molecular hydrogen formed by a chemical reaction in ahydrogen production zone. During the reaction, the molecular hydrogen isdissociated and passed as ionic hydrogen to the other side of themetallic foil from which it is withdrawn, thereby removing hydrogen fromthe hydrogen production zone. The concept of the '485 patent above isrestricted to the use of a metal foil as the hydrogen separator. Foilseparators have been proven to be difficult to achieve in practice. Forexample, they do not provide for a reliable structure which can operatefor a reasonable time at high efficiencies.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to overcome thedrawbacks and limitations of the prior art.

Another object of the present invention is to provide a membrane layersuitable for use in an electrochemical autothermal reformer.

Yet another object of the present invention is to provide anelectrochemical autothermal reformer which is reliable and efficient.

The membrane layer of the present invention is a composite of ceramicand metallic components. It requires a mechanical support in the form ofa reticulated alumina monolith preferably with a pore density of 80pores per inch and an average pore size of about 250 micrometers. Theentire monolith is catalyzed with a noble metal such as platinum toenhance the partial oxidation, shift and steam reforming processes. Alayer of alumina (Al₂ O₃) ceramic substrate with a theoretical pore sizeof 2 micrometers at a thickness of about 0.008" is formed over themonolith. A suitable thickness range for the substrate is from about0.005" to 0.010". An electrochemical cell--a thin solid oxide layer,such as cerium oxide or tungsten oxide used to selectively transportboth protonic hydrogen and associated electrons from the monolith sideof the electrochemical autothermal reformer to the anode side of thereactor is formed over the alumina substrate. The function of this layeris to support a thin ceramic electrolyte cell which could not besupported by the monolith. A suitable thickness for the oxide layer isfrom about 4 to 8 microns, with a 6 micron layer providing satisfactoryresults.

A thin, non-continuous layer of catalyst, such as platinum, is depositedon both sides of the electrochemical cell to promote the dissociation ofmolecular hydrogen to protons and electrons on the anode side andrecombination of protonic hydrogen and associated electrons to molecularhydrogen on the cathode side of the electrochemical cell. The catalystlayer may be up to about 100Å thick with a layer of about 25 Å providingsatisfactory results

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencenumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the membrane layer of the presentinvention.

FIG. 2 is an exploded view (or blow up) of the membrane layer shown inFIG. 1.

FIG. 3 is an electrochemical autothermal reformer employing the membranelayer of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a separator membrane suitable for use in thepresent invention is shown. A separator membrane layer 10 includes amonolith catalyst support 12 which is overlaid with an alumina (Al₂ O₃)substrate 14 which provides a gas permeable support layer for a metaloxide membrane or mixed ion conductor layer 18. The metal oxide layer 18is sandwiched between catalyst layers 16 and 20, respectively. Catalystlayer 16 functions to promote the dissociation of H₂ to 2H⁺ and 2e⁻ (ordissociation of hydrogen molecule to protons and electrons) whilecatalyst layer 20 functions to promote the recombination of 2H⁺ +2e.⁻ toH₂. The arrow at the bottom on FIG. 1 indicates the direction of gasflow through the membrane, while the arrow at the top indicates theremoval of hydrogen from the production zone through catalyst layer 20.

The following example illustrates one embodiment of a process for makinga castable alumina substrate layer for use in the present invention.

EXAMPLE 1

Material is purchased from Cotronics Corporation (part number Rescor 780Liquid Alumina; 3379 Shore Parkway; Brooklyn, N.Y. 11235) as a twocomponent system. The components are alumina powder and liquidactivator. The liquid activator is a water based alumina colloidsolution.

Step 1:

The powder is sifted through a screen so that a particle no larger than0.009" in diameter will pass through the screen. The resultant powder isthen sifted again through a size 86 polyester screen. This secondsifting step ensures that the alumina particles will pass through thescreen during the actual silk screen application process.

Step 2:

The sifted material is then mixed in the required 100 parts powder to22-26 parts activator and degassed in a 29" Hg vacuum.

Step 3:

A stainless steel plate is prepared by covering it with a sheet of finepore paper. The paper is pre-soaked with water and stretched to removeany large wrinkles. The pre-soaking procedure prevents the capillaryaction of the examination paper from pulling the activator away from thealumina powder.

Step 4:

A number 86 polyester screen is laid down over the pre-soaked, stretchedmedical examination paper and the mixed alumina poured on top of thescreen. A silk screen squeegee is used to pull the alumina over thescreen and force it through the mesh and onto the pre-soaked medicalexamination paper (for a uniform layer, multiple passes may berequired). The diameter of the threads that make up the screen controlthe theoretical thickness of the layer. A number 86 polyester silkscreen has a thread diameter of 100 micrometers or slightly under0.008".

Step 5:

Since the medical paper can be easily handled with the alumina layer inthe uncured state, it is lifted off of the platen and wrapped around themonolith. Pressure is then gently applied to promote adhesion of thealumina layer to the monolith. The alumina layer (with paper stillattached) on the monolith is then allowed to cure for at least 24 hours.During the room temperature cure, light, uniform pressure is appliedover the area of alumina application to prevent warpage during drying.

Step 6:

After the 24 hour room temperature cure, the monolith with the curedalumina layer (with paper still attached) is placed in a furnace andramped up to 100° C. for 2 hours to drive off any surrounding water. Thelight, uniform pressure applied over the entire alumina area (mentionedin Step 5) is applied throughout the elevated temperature post-cureprocess to minimize warpage.

Step 7:

After 2 hours at 110° C., the furnace is ramped up to 250° C. and heldfor 1 hour to burn off the medical paper.

Step 8:

After the medical paper burn-off, the furnace temperature is increasedto 950° C. and held there for 2 hours to impart extra strength to thecastable alumina.

Step 9:

The entire part is then allowed to slowly cool down to room temperatureover a 24-hour period.

Step 10:

The cooled monolith with substrate layer part is then removed from thefurnace and excess ash from the medical paper burn-off process isremoved by light brushing or compressed air. The entire process resultsin a near uniform layer of porous alumina with a thickness ofapproximately 0.008".

EXAMPLE 2

The formation of the electrochemical cell in the form of a thin tungstenoxide layer (WO₃) is carried out by conventional RF sputtering. Theoxide layer may also be formed by conventional techniques known to theart such as chemical vapor deposition, spin coating and dip coating.

The formation of this non-continuous layers of catalyst material on bothsides of the electrolyte layer is carried out by depositing platinumlayer 16 on the alumina substrate 14 by conventional RF puttering andthen by sputtering the electrolyte layer 18 on top of the catalyst layer16. Finally, a catalyst layer 20 is sputtered on top of the electrolytelayer 18.

Referring to FIG. 3, an EATR (electrochemical autothermal reformer) 200includes an ATR (autothermal reformer) 210 joined to a hydrogen gasspace or anode supply region 230 by membrane layer 10. A fuel containingmolecular hydrogen, such as a hydrocarbon or ammonia, enters ATR 210through an inlet 240. Membrane layer 10 promotes dissociation of thehydrogen in ATR 210 as long as the partial pressure of the hydrogen inATR 210 is greater than the partial pressure of the hydrogen in thehydrogen gas space or anode supply region 230. Reformed fuel orreformate or reaction products from the fuel reformer, ATR 210, less thehydrogen transferred through the membrane layer is exhausted through anexhaust port 250 and burned.

Either the reformer side of EATR 200, i.e., ATR 210, is pressurized orthe hydrogen side, i.e., anode supply region 230, is eluted with a gasor vapor such as nitrogen or water. One method of ensuring that thehydrogen partial pressure in anode supply region 230 is lower than thehydrogen partial pressure in ATR 210 is disclosed in a copendingapplication filed concurrently herewith entitled "FUEL CELL POWER PLANTWITH ELECTROCHEMICAL AUTOTHERMAL REFORMER" (Attorney Docket No. 269-007)and incorporated herein by reference.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

What is claimed is:
 1. A hydrogen forming and extraction processconsisting of:conducting in a hydrogen production zone a chemicalreaction forming mixed gases comprising molecular hydrogen and otherreaction products; contacting one side of a composite membrane layer,which includes WO₃ as a mixed ion layer sandwiched between two catalystlayers of particulate platinum, with said mixed gases in said hydrogenproduction zone; with one of said catalyst layers functioning to promotethe dissociation of H₂ to protons and electrons and the second saidcatalyst layer functioning to promote the recombination of 2H⁺ +2e⁻ →H₂; dissociating said molecular hydrogen to ionic or protonic hydrogen andassociated electrons on said one side of said membrane layer, passingsaid ionic hydrogen through said membrane layer to another side of saidmembrane layer and recombining there with associated electrons to formmolecular hydrogen; and withdrawing hydrogen from said another side ofsaid membrane layer thereby removing hydrogen from said hydrogenproduction zone.
 2. The hydrogen forming and extraction process of claim1 wherein said WO₃ layer is about 760 nanometers thick.
 3. The hydrogenforming and extraction process of claim 1 wherein said membrane layerhas a mechanical support selected from the group consisting ofperforated metal, expanded metal, porous metal, porous ceramic, andmixtures thereof.
 4. The hydrogen forming and extraction process ofclaim 1 wherein said hydrogen production zone is maintained at atemperature between about 800° to about 1400° F.
 5. The hydrogen formingand extraction process of claim 1 wherein said chemical reactioncomprises a steam/methane reforming reaction: CH₄ +2H₂ O=CO₂ +4H₂.